The present invention relates to an electrophotographic image forming apparatus using a continuous form recording medium.
Conventionally, there is known an electrophotographic image forming apparatus, such as a copy machine or printer that employs an electrophotographic image forming process. In the electrophotographic image formation apparatus, the uniformly charged photoconductive material provided on the surface of a photoconductive drum is exposed to light that carries image data to form a latent image. The latent image is developed by adhering toner thereto (a toner image is formed), and the toner image is transferred onto a recording medium and fixed.
Some of the electrophotographic printers print images onto a so-called fanfold recording sheet which is a continuous recording sheet provided with perforated tear lines, which are defined at the portions to be folded (hereinafter, this continuous form recording sheet is simply abbreviated as a continuous sheet). The continuous sheet can be easily cut off at the perforated tear lines.
Incidentally, in the electrophotographic image forming apparatus, a so-called heat roll fixing is generally employed.
In the heat roll fixing, a pair of fixing rollers comprising a heat roller heated to a high temperature and a press roller are arranged in parallel. The recording sheet, carrying an unfixed toner image thereon, is positioned between the pair of fixing rollers. The unfixed toner image on the recording sheet is fused by being heated with a heated roller (heat roller) and fixed onto the recording sheet at a fixing unit. The heat roll fixing is advantageous in that an excellent energy efficiency is realized and a fixing speed can be increased.
The fixing unit also functions as a feed mechanism to feed the recording sheet that is positioned between the pair of fixing rollers. Usually, the heat roller is rotatably driven, and the press roller is rotated in accordance with the heat roller. In this case, the feed speed of the continuous sheet by the heat roll fixing unit (e.g., the peripheral speed of the heat roller) is set to equal the peripheral speed of a photoconductive drum with high accuracy.
In the electrophotographic printer as above, wherein images are printed onto a continuous sheet, and the continuous sheet is fed by the heat roll fixing unit, a problem arises in that so-called skewing occurs (e.g., the continuous sheet proceeds obliquely or windingly) when the photoconductive drum is not accurately disposed in parallel with the heat roller, or there is a difference in the contact pressure between the heat roller and the press roller in the width direction of the continuous sheet. It is very difficult to eliminate these skew factors by improving a mechanical accuracy.
Once skewing arises, the position where the fixing unit (fixing roller pair) takes in the continuous sheet is moved to an oblique direction as the continuous sheet is fed. Finally, a jam occurs.
The skewing occurs with a relatively low frequency at a high printing ratio wherein the photoconductive material has a low potential because the larger area thereof is exposed to light, and further the continuous sheet is in less intimate contact with the photoconductive material (photoconductive drum) because toner exists therebetween.
On the other hand, in a low printing ratio, skewing tends to occur because the continuous sheet is in intimate contact with the photoconductive drum, and the photoconductive drum strongly affects the feed of the continuous sheet. If a proceeding direction of the continuous sheet is slightly displaced from its normal feeding direction, the more the photoconductive drum affects the feeding of the continuous sheet, the more the deflection of the continuous sheet is accumulated to cause skewing.